The purpose of a catalytic converter is to facilitate conversion of pollutant materials in internal combustion engine exhaust, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxides, etc. to carbon dioxide, water, nitrogen and other harmless gases. Conventional catalytic converters utilize a ceramic honeycomb monolith having square, triangular, or circular, straight-through openings or cells, catalyst coated alumina beads, or a corrugated thin metal foil honeycomb monolith. These monoliths are characterized by having a catalyst carried on or supported by the surface, which surface is, in the case of the thin metal honeycomb monolith, typically washcoated with one or more refractory metal oxides, e.g., alumina (gamma), ceria, lanthia, or combinations thereof, and a catalyst. The catalyst is normally a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more of such metals. The catalyst may also be manganese hexa-aluminate developed by Kobe Steel, Ltd. The catalyst catalyzes a chemical reaction whereby the pollutant material is converted to a harmless by-product which then passes through the exhaust system to the atmosphere.
However, this conversion is not efficient initially when the exhaust gases are relatively cold. To have high conversion efficiency, for example, at start-up, the catalyst and the surface with which the exhaust gases come in contact must be at a minimum elevated temperature, e.g., 390 F. for carbon monoxide, 570 F. for volatile organic compounds (VOC) and 1000 F. for methane or natural gas. Otherwise, conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. Once the exhaust system has come to its normal operating temperature, the catalytic converter is optimally effective. Hence, it is necessary for the relatively cold exhaust gases to contact a catalyst with very low thermal inertia of preferably, a hot catalyst to effect satisfactory conversion at start-up.
A polycellular corrugated thin metal honeycomb monolith having a catalyst deposited on the surface thereof is especially adapted to this purpose in that it can be heated readily by electrical means as described in our copending application Ser. No. 07/680,763 filed Apr. 5, 1991 entitled Composite Catalytic Converter, to which reference may be had, and which copending application is incorporated herein by reference thereto. This application specifically discloses alternative means for reinforcing the leading edge of the corrugated thin metal strip useful as heatable catalytic supports.
To achieve rapid heating of the catalyst in a metallic monolith by electrical means, it is necessary to draw a large amount of power from a voltage source or another source of electrical energy, e.g., a battery or a capacitor device, such as the new Isuzu "electric power storage" device developed by Isuzu Motors, Ltd. for a short period of time until the desired catalyst temperature is reached. In an automotive vehicle, for example, this source of electrical energy is usually a 12 volt or 24 volt battery, although a battery system supplying up to as much as 108 volts may be used herein. To accomplish a high power draw on a storage battery system, it has been found that one or more actuatable solid state switches connected in parallel, such as metal oxide semiconductor field effect transistors (MOSFETs), together with means for actuating such devices in unison (a gate driver) may conveniently be used. Such a system enables drawing high power loads for a short period of time sufficient to achieve the desired catalyst temperature of about 650 F. in from 2 to 30 seconds. Reference may be had to our copending application Ser. No. 587,219 filed Sep. 24, 1990 for details of a suitable power control system useful herein.
Reference may be had to U.S. Pat. No. 3,768,982 to Kitzner dated Oct. 30, 1973. In this patent, heat from a centrally located electric heater is transferred by conduction through a monolith catalyst support to heat the catalyst to optimum operating temperature. Reference may also be had to U.S. Pat. No. 3,770,982 to Kitzner dated Oct. 30, 1973 which discloses a central electrically heated core within a ceramic monolith. The heating core is formed of metal sheets, one corrugated and the other flat, coated with alumina and also bearing a catalyst. The metallic core is heated electrically by virtue of its own electrical resistance. However, heating by conduction requires too long a period (a matter of minutes) to be practical in solving the problem of atmospheric pollution at start-up.
Reference may also be had to U.S. Pat. No. 4,711,009 to Cornelison et al dated Dec. 8, 1987 for details of a process for the preparation of a continuous corrugated thin stainless steel strip having a wash coat of alumina (gamma) on at least one surface of the strip, and a noble metal catalyst deposited on the resulting surface thereof. This patent is incorporated herein by reference thereto.
The process described in U.S. Pat. No. 4,711,009 may be modified by including in the apparatus after the initial heat treating or annealing station, an edge overfolding station in which at least one edge is reverse folded one or more times upon the balance of the strip to multiply the thickness along the length of the strip edge. In the preferred configuration, the folded over portion, or hem, is seam welded continuously or intermittantly to the parent foil. Also, the overfolding may occur along both edges of the strip, although for most purposes, only what will become the leading edge of the accordion folded strip or the spirally wound strip requires reinforcement by multiplying the thickness of the strip. The greater thickness of the leading edge also enhances heat transfer. In general, folding of the leading edge prevents flutter due to the inpingement of turbulent, hot gases. Folding at the trailing edge prevent crushing or collapse of the monolith especially in hot cyclic service, and in particular, the hot shake tests. Corrugation is done after the overfolding step. From that point on, the process is essentially as disclosed in the aforesaid patent. For conventional converters, accordion folding of the corrugated thin metal strip may be used. For electrically heatable catalytic converters, a series of layered elongated strips are used. Instead of accordion folding an elongated strip to form the monolith, a strip of predetermined length may be cut from the continuous strip and spirally wound about a central core as described in the aforesaid application Ser. No. 680,763.
Reference may also be had to International PCT publication numbers WO 89/10470 and WO 89/10471 each filed Nov. 2, 1989 which disclose electrically conductive honeycomb catalyst support units useful in automobiles. S-wound cores are disclosed in these publications.
To applicant's knowledge, no reference discloses a corrugated thin metal foil having a reinforced leading or trailing edge formed by overfolding one or both edges. The edge of the corrugated thin metal strip which first comes in contact with the exhaust gases is deemed to be the "leading edge."
In the following description, reference will be made to "ferritic" stainless steel. A suitable formulation for this alloy is described in U.S. Pat. No. 4,414,023 dated Nov. 8, 1983 to Aggens et al. A specific ferritic stainless steel alloy useful in forming the corrugated thin metal strips hereof contains 20% chromium, 5% aluminum, from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium, and praseodymium, or a mixture of two or more rare earths, balance iron and steel making impurities.
In the following description, reference may be made to fibrous mat or insulation. Reference may be had to U.S. Pat. No. 3,795,524 dated Mar. 5, 1974 to Bowman for formulations and manufacture of ceramic fibers and mats useful herein. One such material is currently available from 3-M under the registered trademark "INTERAM."
In the following description reference will also be made to brazing foil. This foil is cast and/or rolled to about 0.001" to about 0.003" thick. It is desirably a nickel-chromium-boron-silicon brazing alloy analyzing 75% to 83% nickel with a liquidus temperature of 2100 F. to 2300 F. Other nickel-containing brazing alloys contain 7% to 14% chromium, 3% to 4.5% iron, 3.5% to 4.5% silicon, 2% to 3% boron, balance nickel and having a liquidus temperature above about 2100 F. may also be used. Phosphorus in the alloy is to be avoided where platinum is used as the catalyst. Such alloys are currently available from Allied Metglas Products in Parsippany, N.J.
Many millions of automotive vehicles are equipped with catalytic converters, but virtually all are subject to start-up emissions of what, in at least one state, has been determined to be an unacceptable level. Anticipatory elevation of the catalyst temperature to an optimum operating level before start-up is expected to be mandated for many, if not all cars.
The metal monolith devices utilize, in the preferred embodiments, an elongated thin metal strip with reinforced edges, corrugated in such a manner as to be nonnesting when accordion folded, or when spirally wound about a central core. The corrugations may be, therefore, herringbone, truncated herringbone (as shown in FIG. 4 of U.S. Pat. No. 4,838,067 to Cornelison dated Jun. 13, 1989, chevron shaped or sinusoidally shaped, having a V-cross section with the apices truncated or rounded to reduce stress. The corrugations may be straight-through according to a variable pitch scheme such as described in U.S. Pat. No. 4,810,588 to Bullock and Whittenberger dated Mar. 7, 1989. The sloping sides of the herringbone, or chevron patterned corrugations define an angle of from 3 to 10 degrees, e.g., 5 degrees, to a line normal to the edges of the strip.
There is a tendency for the leading edges, particularly, of corrugated thin metal foil strips at high gas space velocities on the order of 1,000,000 volume/volume/hour to roll over and induce destruction of the catalytic converter unit. To overcome this problem, it has now been found that reinforcing the leading edge and optionally the trailing edge of the corrugated thin stainless steel foil strip by overfolding an edge of the foil strip a distance of from 5% to 40% of the width of the final strip thereby doubling or quadrupling the thickness of the foil in the edge region(s) avoid this problem.